Nowadays, use of integrated circuit (IC) devices is ubiquitous. Some of these IC devices provide both analog and digital functions in a single package. One such IC device may be a digital signal processor (DSP), which typically includes analog amplifiers and digital circuits for processing an input signal.
It is desirable to design an IC device to have a low power consumption especially when the device is employed in a battery powered apparatus, e.g., a wireless telephone handset, which is common. Since the battery in such an apparatus has a limited capacity, the lifetime of the battery before replacement or recharging thereof increases as the power consumption of the IC device decreases. To reduce power consumption in an IC device, it has been proposed that the voltage of the power supply to the IC device is to be reduced from about 3 volts presently to about 2 volts in the near future.
A typical analog amplifier is realized in an IC device using field effect transistors (FETs), e.g., metal-oxide semiconductor FETs (MOSFETs). Such an amplifier comprises a current-mirror circuit, an output stage, and a differential amplifier which receives inverting and non-inverting inputs to the amplifier, respectively. In the typical amplifier, an FET in the output stage, referred to as an "output bias" transistor, is biased with a constant current dictated by the current-mirror circuit. The drain current of such an FET contributes to the current which drives a load, e.g., a resistive load, at the output of the amplifier.
However, the prior art analog amplifier just described is a class A amplifier which, as is well known, constantly provides a significant amount of current to the amplifier output, regardless of whether it is in a steady state or transient state. Although such a class A amplifier desirably provides substantially linear amplification, and introduces virtually no distortion into the output, especially during the transient state for which the significant amount of output current is required, the power consumption by the amplifier is undesirably high because of its provision of such a significant amount of output current all the time, notwithstanding that the power supply voltage to the amplifier is reduced.